1. Field
One embodiment of the present invention relates to a disk drive, and more particularly to a technique of controlling the magnetic head that includes a spin-torque oscillator configured to accomplish microwave assisted recording.
2. Description of the Related Art
In recent years, the recording density and capacity have been increased in the field of disk drives, the representative example of which is the hard disk drive, owing to the development of giant magnetoresistive (GMR) magnetic heads and perpendicular magnetic recording. Along with the GMR magnetic head and the perpendicular magnetic recording, the high-frequency assisted recoding performed by applying a high-frequency magnetic field to a magnetic disk has been proposed as a technique that may further increase the recording density. (See, for example, U.S. Pat. No. 6,011,664.)
In the high-frequency assisted recording, a high-frequency magnetic field is applied to a selected part of a magnetic disk, at a frequency that is much higher than the frequency of signals to be recorded and is near the resonance frequency of the disk. When applied with the high-frequency magnetic field, the magnetic disk undergoes resonance. As a result, the coercive force (Hc) at that part of the disk surface, which is applied with the high-frequency magnetic field, decreases to half or less the initial coercive force. In this state, a high-frequency magnetic field is superimposed on the recording magnetic field, whereby data can be recorded on the disk having a high coercive force (Hc) and a high magnetic anisotropic energy (Ku). In the method disclosed in the above-identified patent, however, a coil generates the high-frequency magnetic field, which cannot be efficiently applied to the disk at the time of high-density recording.
As a technique of solving this problem, a method is proposed, in which a spin-torque oscillator (hereinafter referred to as STO, in some cases) is used as a source of a high-frequency magnetic field. (See, for example, US-A 2005/0023938 and US-A 2005/0219771.) In this method, a direct current is supplied to the STO through an electrode, making the STO generate a high-frequency magnetic field.
A spin-torque oscillator may be incorporated into a magnetic head. High-density magnetic recording can then be effectively performed only on that part of a magnetic disk, where the high-frequency magnetic field the spin-torque oscillator generates is superimposed on the recording magnetic field the magnetic head generates.
In any disk drive, a write current (recording current) is supplied to the magnetic head. The write current is inverted in polarity in accordance with the data to write on the magnetic disk. The spin-torque oscillator has a delay time (oscillation delay), i.e., time that elapses until it starts oscillation after the write current has been inverted in polarity. Consequently, the data transfer rate of the disk drive increases. If magnetization inversion is repeated at intervals shorter than the oscillation delay to write data on the magnetic disk, the spin-torque oscillator will fail to operate appropriately, possibly causing recording errors.